WASHINGTON >> A decade before he became President Donald Trump’s health secretary, the environmental lawyer Robert F. Kennedy Jr. appeared on a talk show hosted by Dr. Mehmet Oz to promote his latest book, “Thimerosal: Let the Science Speak.”

The book, published in 2014, explored an obscure mercury-based preservative, thimerosal, that was removed from most vaccines, but not flu vaccines, more than two decades ago. Oz noted that the Centers for Disease Control and Prevention had deemed the vaccines that still contained thimerosal “safe and effective” and said they did “not present a public health risk.”

Kennedy did not buy it. “We found 500 peer-reviewed studies,” he insisted. “Virtually every one of them said that thimerosal is a potent neurotoxin that should not be in vaccines.”

On Thursday, the new members of the CDC’s Advisory Committee on Immunization Practices, hand-selected by Kennedy after he fired all 17 members of the previous panel, decided it would no longer recommend annual flu shots that contain it. Thimerosal’s appearance on the committee’s agenda in the first place shocked public health leaders, who have long considered the matter settled.

But it was not a surprise to people who have followed Kennedy closely. Thimerosal started Kennedy down a path of questioning vaccine safety, and Thursday’s vote was the culmination of a long personal journey. It offers a window into how, as secretary, he is pursuing his own passions and installing old allies in positions of influence.

“He’s got a big passion for this subject, and he knows this probably better than anybody,” said Eric Gladen, who featured Kennedy in his 2014 film, “Trace Amounts,” which espoused a link between thimerosal and autism.

Critics say that in resurrecting an old controversy, Kennedy could brew mistrust rather than ease it. Numerous studies, including a 2004 report by the Institute of Medicine and a 2010 review of the medical literature, have rejected a link between the preservative and autism. Oz, who now runs the Centers for Medicare and Medicaid Services, noted on his 2014 show that any link had been “ultimately discredited.”

But Kennedy embraced the theory, and traveled the country with Gladen to promote the film. At the time, Gladen headed an organization called the World Mercury Project. He joined forces with Kennedy, who later took over World Mercury Project and turned it into Children’s Health Defense, the advocacy group Kennedy led until he ran for president.

“It’s unbelievable, when I sit down and think about it,” Gladen said. “There is nothing better that could have possibly happened for his book, the film, our organization, to get him into a position where he now oversees the CDC and the FDA, and oversees what I consider to be a massive disaster and to try to make it right.”

When Kennedy became health secretary, he installed David Geier, who promoted the theory that mercury in vaccines is linked to autism, as a special government employee in the Department of Health and Human Services, to work on a study of vaccine safety.

Geier was fined in 2012 by Maryland regulators, who found he had practiced medicine without a license. Kennedy has defended Geier, saying the finding was reversed by a court.

Now Kennedy has enlisted another ally in the thimerosal battle, Lyn Redwood, president emeritus of Children’s Health Defense, as a special government employee. Redwood, a nurse, is the parent of a child with autism and a founder of Safe Minds, a nonprofit that worked in the early 2000s to reduce children’s exposure to all sources of mercury.

In 2004, Redwood published an article arguing that the theory that thimerosal caused autism was a “plausible hypothesis that should not be dismissed.” She was invited as an expert to Thursday’s advisory committee meeting, where she made a presentation to the panel arguing that thimerosal should be removed from flu vaccines.

Notably, Redwood did not make an aggressive case on Thursday that the preservative caused or was linked to autism. Rather, she focused on its role as a “neurotoxin,” which she said posed a particular risk to pregnant women and fetuses.

At one point, she invoked a study that “did not support a causal association between early exposure to mercury from thimerosal-containing vaccines” and “deficits in neuropsychological functioning” in 7- to 10-year-olds. The study did, however, find an association with tics, which “can be very debilitating,” she told the panel.

Dr. Paul Offit, a vaccine expert at Children’s Hospital of Philadelphia who served on the advisory committee from 1998 to 2003, remembers Redwood protesting the CDC at that time, when health experts were originally debating thimerosal’s risks. He objected to an advocate making a scientific presentation to the advisory committee.

“It wasn’t clear to me that she was saying autism anymore, like she did 25 years ago,” he said of her presentation Thursday, adding, “It’s like Whac-a-Mole. They just keep moving onto the next thing.”

The panel voted 5-1 on Thursday to stop recommending flu vaccines that contain the preservative. It was unclear how manufacturers would respond, and how the recommendation might affect access to flu vaccines. Some flu vaccines are already available without thimerosal.

Thimerosal emerged as an issue in 1997, after Congress, concerned about all forms of mercury exposure in children, asked the Food and Drug Administration to examine mercury in foods and drugs. The following year, Andrew Wakefield, a British doctor, published a study that raised alarms by suggesting vaccines were linked to autism. The Wakefield study, which was eventually debunked, focused on the measles, mumps, rubella vaccine, which did not contain thimerosal.

But a theory took hold that thimerosal in vaccines was linked to autism.

In mid-1999, the CDC and the American Academy of Pediatrics put out a joint statement calling for thimerosal’s removal from vaccines, out of an abundance of caution, and manufacturers largely heeded the call.

Dr. Walter Orenstein, who ran the CDC’s vaccination programs at the time, said experts decided to recommend the removal of thimerosal before the FDA study was completed.

“None of us have ever thought autism would become the allegation, because mercury had never been linked to autism,” Orenstein said in an interview. “But the big issue was, if we waited for science to tell us whether there was harm or no harm, we could have harmed people.”

Kennedy, then working as an environmental lawyer, had been focused on removing mercury in waterways. As he has often told the story, mothers of children with autism approached him during public appearances, asking him to look into mercury in vaccines.

“They would say to me in a very respectful but also kind of vaguely scolding way, ‘If you’re really interested in mercury exposures to children, you need to look at vaccines,’” Kennedy later told an audience at Hillsdale College.

By the time he got involved in the debate, the pharmaceutical industry had already removed thimerosal from all vaccines except those for flu. Experts said it was feasible for the makers to remove the preservative for vaccines in the United States, a wealthy country that could afford more expensive single-dose vials. Influenza vaccines, produced in high volume for mass vaccination campaigns, were the exception.

In 2005, Rolling Stone magazine and Salon published an article by Kennedy, headlined “Deadly Immunity,” that blamed thimerosal in vaccines for fueling the rise in autism. Years later, Salon retracted the article. Kennedy insisted that Salon had caved to pressure from government regulators and the pharmaceutical industry.